1. Field of the Invention
The present invention relates in general to inflatable passive restraint systems for use in vehicles for restraining the movement of a seated occupant during a collision. In particular, the present invention relates to an improved inflatable air bag having a variable volume for proper inflation over a wide range of generated gas volumes.
2. Description of the Related Art
In recent years, passive restraint systems, particularly those restraint systems incorporating inflatable bags or cushions (commonly referred to as "air bags") have been used with increasing frequency in automobiles. In such air bag systems, one or more air bag modules are stowed in respective storage areas within the passenger compartment of the vehicle. Upon actuation, the air bags are deployed into the vehicle passenger compartment through openings in the vehicle interior.
This deployment is effected by filling or inflating the air bag proper using gasses generated by, or released from, an inflator unit. The inflated air bag thus acts as a cushion for the passenger. In this role as a cushion, the proper inflation is important. For example, if the bag is underinflated, an insufficient pressure is applied to appropriately stop the passenger's forward movement. Conversely, if the bag is overinflated, the bag is too rigid and too much pressure will be applied, such that the bag itself causes injury. To reduce the possibility of overinflation, and to provide better cushioning, various vents are typically provided in the bag for expelling the inflating gas.
In addition to inflation, air bag size is also an important consideration. Specifically, the air bag is filled with gas upon deployment to fill a certain volume, and to extend a certain distance toward the passenger (referred to herein as length). During cushioning of the passenger to halt forward movement, the air bag is partially deflated, reducing its volume. This reduction in volume is primarily due to compression of the bag in the direction of the passenger, i.e., lengthwise. As such, the bag must have a length which permits it to partially compress when halting the passenger's movement, yet still provide a barrier between the passenger and the steering wheel, dashboard, etc. when the movement is halted.
To complicate this situation, the proper amount of inflation for an air bag will vary with several factors. For example, the severity of the collision, and thus the necessary deceleration of the passenger will vary for each collision. In this regard, less inflation is needed for relatively minor collisions, while more inflation is needed for severe collisions. The severity of the collision also determines the size of the air bag, specifically in the direction of the passenger or length. For minor collisions, the length need not be as great as for severe collisions.
Another factor (at least for certain types of inflator units) is ambient temperature. For the most common types of contemporary inflator units, the inflator will produce an amount of gas which varies proportionally with ambient temperature. As such, if the temperature is low (e.g., 5.degree. C.), less gas is produced, and if the temperature is high (e.g., 30.degree. C.), more gas is produced.
These factors are of course taken into account during air bag design. For example, the length of the air bag is designed to be the largest necessary for severe collisions, and the volume is designed to provide proper inflation pressure for a severe collision even at cold temperatures. The possibility of excess pressure for a lesser collision is alleviated by the design of the vents in the bag, or additional vents in the gas generator.
While this arrangement is acceptable, it has been desired in the art to permit a bag response to vary with these (or other) factors to provide a more optimal response. To this end, U.S. Pat. No. 5,048,863 to Henseler et al. discloses an air bag system having a gas generator which produces proportionally larger amounts of gas with increasing severity of collision. This of course increases internal pressure in the bag for more severe collisions.
This patent also discloses that the bag may have a variable volume. This is achieved by taking a bag, folding an exterior wall of the bag upon itself, and then providing break-away stitching to maintain the fold. This stitching will maintain the fold at lower pressures, but fails at the higher pressures produced for severe collisions. As such, the volume and length of the bag are increased for more severe collisions.
While this arrangement does permit the bag response to vary in order to improve performance, there are drawbacks to this approach. Specifically, the folding is produced near the forward face of the bag, close to or including the contact area with the passenger. Passenger contact with these folds is less desirable than passenger contact with a smooth unbroken surface. Additionally, the folds in this patent are not stitched across their length, but are only "tacked" at discrete locations. This will result in the fold being inflated along with the main section of the bag. This would of course tend to distort the bag shape and again provide a less than desirable surface for passenger contact.